Radiofrequency (RF) catheter ablation of either the "slow" or "fast" AV nodal pathways can cure AV node reentrant tachycardia (AVNRT) and also modify ventricular response to atrial flutter and fibrillation. However, neither the tissues nor mechanisms underlying these arrhythmias are known. Recently, we demonstrated that two separate atrial circuits overlap in the AV junction region: components of the "muscular valvular apparatus", the "circumferential and the perpendicular laminae," which also form the inferior medial atrial wall; and, the collagen encased "multilimb input" to the AVN: the "atrionodal bundles (ABs) and the proximal AV bundle (PAVB)," which is outside of the medial atrial wall epicardium. We showed that each tissue possess unique extracellular (EAP) and transmembrane action potentials (TAP) and transmission properties; EAPs from the atrial and specialized tissues appear side-by-side in traces made at sites where the atrial and specialized tissues overlap; and atrial EAPs and contractions ceased with exposure to high potassium. HYPOTHESIS: A specialized multilimb AVN input with unique histologic and conduction properties is present in human and dog heart. SPECIFIC AIMS are to determine: 1) the position of the ABs and the PAVB in human heart; 2) the myocyte evoking the AB potential and its electrical pathway after iontophoresis of Lucifer Yellow (LY); 3) transmission properties of the ABs during program stimulation of the ABs and SAN by evalulation of the SAN-AB intervals; 4) each ABs role in AVN activation by alterations in the SAN-AVN interval due either to selective ablation of LY-fiUed myofibers or to transection of the AB/PAVB junctions. METHODS: Electrical potentials will be recorded using simultaneous (a) stationary catheter electrodes at the SAN, 3ABs, PAVB, and AVN to monitor electrical coupling, (b) wire electrodes to localize injection and recording sites, responses to photoablation, (c) multielctrode array plaque, (d) 1 percent LY or 3M KCI miropipet electrodes for recording and dye injection using current pulses. Ablations will be made using blue light and scalpel blades. The anatomy, LY pathways, and effects of photoloysis will be evaluated by 3D analysis and reconstructions. Alterations in morphology of electrical potentials, and conduction intervals will be confirmed via timing in the SAN trace and correlation of EAPs and TAPs. These studies are expected to provide a basis for evaluating transmission, arrhythmogenesis, and drug interactions at the tissue level.